1. Field of the Invention
The present invention relates to a surface acoustic wave filter utilizing shear horizontal type (SH type) surface acoustic waves, and more particularly, to a transversal-coupled resonator type surface acoustic wave (hereinafter referred to as SAW) filter.
2. Description of the Related Art
Transversal-coupled resonator type double mode SAW filters utilizing Rayleigh waves are known. An example of this type of lateral coupling type SAW filter will be described with reference to FIG. 6.
FIG. 6 is a simplified plan view for explaining a configuration of electrodes of a conventional transversal-coupled resonator type SAW filter using Rayleigh waves. A SAW filter 31 has a structure wherein first and second interdigital transducers (hereinafter abbreviated to read "IDTs") 32 and 33 are provided on a piezoelectric substrate (not shown) and grating reflectors 34, 35, 36, 37 are provided on both sides of the IDTs 32 and 33.
The IDT 32 includes a pair of comb-shaped electrodes 32a, 32b each having at least one electrode finger. The comb-shaped electrode 32a and the comb-shaped electrode 32b are arranged such that their electrode fingers are interdigitated with each other. Similarly, the IDT 33 includes a pair of comb-shaped electrodes 33a and 33b.
The space between adjacent electrode fingers of the IDTs 32 and 33 is set to be .lambda./4, and the width of each electrode finger is set to be .lambda./4. .lambda. represents the wavelength of a surface acoustic wave to be excited in the surface acoustic wave filter. The comb-shaped electrode 32b of the IDT 32 and the comb-shaped electrode 33a of the IDT 33 share a common bus. The comb-shaped electrode 32a of the IDT 32 is the input electrode and the comb-shaped electrode 33b of the other IDT 33 is the output electrode of the SAW filter 31. The comb-shaped electrodes 32b and 33a are grounded.
When an input voltage is applied to the IDT 32, surface acoustic waves are excited at the IDT 32 and the excited surface acoustic waves are propagated in a direction perpendicular to the direction in which the electrode fingers extend and are reflected between the reflectors 34 and 35 to become a standing wave. An output that is based on this standing wave is obtained at the IDT 33.
In the SAW filter 31, a surface acoustic wave of a symmetric mode indicated by the solid line S in FIG. 6 and a surface acoustic wave of an asymmetric mode indicated by the solid line A are generated. Thus, the SAW filter 31 operates as a transversal-coupled resonator type double mode SAW filter.
While the first and second IDTs 32 and 33 of the SAW filter 31 shown in FIG. 6 are coupled in a direction perpendicular to the propagating direction of surface acoustic waves, i.e., in a lateral direction, a configuration is also known in which a plurality of SAW resonators having the above-described first and second IDTs 32 and 33 are further coupled in the lateral direction. In a SAW filter 41 shown in FIG. 7, transversal-coupled resonator type SAW filters having IDTs 32 and 33 are transversally coupled to each other in the form of three stages. The configuration of each of the IDTs and reflectors is the same as that in the SAW filter 31 shown in FIG. 6.
In the SAW filter 41, a comb-shaped electrode of IDT 32 of the upper stage is used as the input end as illustrated, and an outer comb-shaped electrode of an IDT 33 of the SAW filter at the final stage is used as the output electrode. Conductive patterns 42 and 43 connected to the ground potential are provided between the stages, and the conductive patterns 42 and 43 are connected to a bus bar which is connected to the ground potential of the SAW filter at each stage.
As described above, in a conventional transversal-coupled resonator type SAW filter utilizing a Rayleigh wave, reflectors 34 through 37 have been required on both sides of IDTs 31 and 32 in order to produce a standing wave. Such reflectors have a structure in which a multiplicity of electrodes fingers are short-circuited or open-circuited. Therefore, it has been necessary to increase the dimensions of the substrate according to an amount required for providing the reflectors 34 through 37.
Further, in the case of conventional SAW filters 31, 41, there has been a problem in that only a filter having a narrow band can be configured because the electromechanical coupling coefficient of a piezoelectric substrate is not so great.
Accordingly, there has been a demand for a transversal-coupled resonator type SAW filter which can be formed with smaller overall dimensions and which has wider band characteristics.